This invention relates to optical systems and, more particularly, to the testing of the quality thereof.
The traditional method of evaluating the quality of an optical system is to perform a knife edge test. This test is used for any optical system which forms an image in at least one plane. Fundamentally, a knife edge is caused to traverse in the focused plane of light from an optical system under test. If the system does not have imperfections, an observer will see a uniform decrease in illumination as the knife edge moves through the plane. If the optical system under test has imperfections, these will show up as light and dark zones.
Such knife edge systems are not only useful in testing the quality of optical systems but also useful for making them. For example, parabolic optical surfaces are typically made by opticians by starting with a spherical surface and figuring the sphere into a parabola using the knife edge test. The test involves the use of an auxiliary optic (large flat, second large parabola, etc.). A knife edge illuminator is then placed at the focal point. The patterns which are visible show up imperfections, that is, where material need be removed to obtain the parabolic surface.
The knife edge tests are not only useful for testing the quality of optical surfaces such as mirrors but for any optical system. For example, lens systems (designed with one infinite focus) may be tested by using an optical flat to return the light therethrough with the knife edge at the rear focal point.
A typical knife edge test system like that currently being used in the optics industry is illustrated in FIG. 1. The system is shown (for simplicity) testing a single spherical optical surface 10 (in reflection). Light from a source 12 such as light bulb, is collected by a lens 14 and focused on a pinhole 16 which is mounted on the side of a cube splitter, 15. The light diverges from pinhole 16 and is reflected toward the tested optic 10 by partially reflecting surface 20. A series of three mutually orthogonal adjustments (represented in FIG. 1 by the single adjustment 19) allows the positioning of the system so that the apparent optical position of the source (pinhole 16) is at the center of curvature (of focal point in the case of an optic with an infinite conjugate). At this point the optical energy is returned back upon itself and forms an image of pinhole 16 at point 30 on the back of the splitter cube 15. On the back of cube 15 is knife edge 22 whose edge is just outside point 30. At this stage all the light passes by knife edge 22 and into the observers eye, 24. The observer slowly turns screw 19 pushing knife edge 22 through the focal plane. If the optic is perfect the surface 10 appears to go dim uniformly. If surface 10 is not perfect various zones will appear lighter or darker (lighter one side darker on the other), and based on these light and dark zones, material can be removed from surface 10 to drive it toward perfection.
If the optic is imperfectly formed or the knife edge is out of the focal plane, or the system is maladjusted, the surface of the optic will appear with some regions bright and some dark and the observer can tell what is imperfect in the optic or the alignment.
This system works well for optics of moderate aperture (e.g. F/3=0.164 N.A.) and can be used to about f/2 (N.A.=0.243) with difficulty. For optical systems of larger numerical aperture this system is unsatisfactory because it is difficult to fill the entire mirror 10 with light using lenses of reasonable size. Additionally, when testing large numerical aperture systems in this manner, the human eye (f/3) isn't sufficiently fast to accept all of the rays from the mirror 10 under test, particularly those rays from the edges of the mirror and therefore, one cannot test the entire mirror.
For testing mirrors with a low f-number or high numerical aperture other means are provided. One method of avoiding the problem is to provide an auxiliary optical element which is slow intermediate the knife edge and element under test. This works if the element under test is relatively small, however, for large systems the cost of generating the auxiliary optic may be prohibitive.
A second solution to the problem is to employ a laser unequal path interferometer. This works quite well if the system under test is already of very high quality (not more than a few waves from diffraction limited.). If the system is not of this quality, no information can be obtained. This solution is also very expensive because of the alignment requirements internal to the instrument.
Accordingly, it is an object of this invention to provide an improved knife edge system.
It is another object of this invention to provide a system for testing a high numerical aperture optical elements.